I sprayed my firewall yesterday and for the first time I noticed that my pressure was swinging from 29 down to 22 until the compressor refilled. I have 20' of 1/2" copper feeding my regulator that is set at 75 psig on the end of my Devilbiss qc3. I have prevost 3/8" hi flow fittings and 3/8" ID 50' hose. It seems like there is to much pressure drop from everything already but I thought that what ever the regulator was set at it would hold? my compressor range is 135-110 or so. any ideas? just a cheap regulator?

Compressed air is measured in two capacities, pressure and volume. Pressure is measured as Pounds per Square Inch or PSI. Volume is measured in Cubic Feet per Minute or CFM (sometimes referred to as SCFM). These are two different things and should not be confused one with another.

Air tools, including spray guns, will have the required amount of air (CFM) given at a specific operating pressure (PSI) for them to run properly. This means that your compressor must be capable of producing that much or preferably more than this amount. For example here are the Specifications for the DeVilbiss Tekna Copper:

Key Specs

Air Cap 7E7 High Efficiency Air Cap (as efficient as HVLP!)
Fluid Tips 1.3mm needle/nozzle set as well as a 1.4mm needle/nozzle set
Materials All basecoats (inc. waterborne), all clearcoats, all single-stage
Air Pressure Input pressure: 16 to 30 PSI
Spray Pattern 12 ½ inch
Air consumption 9CFM to 13CFM


Notice that two different specs are listed, Air Pressure and Air Consumption. To be safe in calculating air supply I suggest you take the high numbers so for this gun to operate properly your air compressor must produce 13 CFM at 30 PSI.

Another concern is the run time of the compressor so sizing your compressor to handle at least 15 CFM at 40 PSI is recommended. This is especially true if you plan to use an air sander or sandblaster as they required large volumes of air as well.

If your compressor doesn't have the output capacity to handle your gun it will produce the scenario you describe. It will start out working fine but shortly after it will not as the compressor cannot keep up.

What is the CFM of your compressor and what is the required CFM of your gun?